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Geo logical Survey I Departm ent of th e Interi o r
eologic Time
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Cover photograph:
Chromolithograph of the Grand Canyon, by William Henry Holmes, printed in 1882.
Geologic Time
by William L. Newman
Geologic Time
The Earth is very old-4112 billion years or more-according to recent
estimates. This vast span of time, called geologic time by earth scientists, is
difficult to comprehend in the familiar time units of months and years, or even
centuries. How then do scientists reckon geologic time, and why do they
believe the Earth is so old? A great part of the secret of the Earth's age is locked
up in its rocks, and man's centuries-old search for the key Jed to the beginning and nourished the growth of geologic science.
Mankind's speculations about the nature of the Earth inspired much of the
lore and legend of early civilizations, but at times there were flashes of insight. The ancient historian Herodotus, in the 5th century B.C., made one of
the earliest recorded geological observations. After finding fossil shells far
inland in what are now parts of Egypt and Libya, he correctly inferred that
the Mediterranean Sea had once extended much farther to the south. Few
believed him, however, nor did the idea catch on. In the 3rd century B.C.,
Eratosthenes depicted a spherical Earth and even calculated its diameter and
circumference, but the concept of a spherical Earth was beyond the imagination of most men. Less than 500 years ago, sailors aboard the Santa Maria
begged Columbus to turn back lest they sail off the Earth's "edge." Similar
opinions and prejudices about the nature and age of the Earth have waxed
and waned through the centuries. Most people, however, appear to have traditionally believed the Earth to be quite young-that its age might be measured
in terms of thousands of years, but certainly not in millions.
The evidence for an ancient Earth is concealed in the rocks that form the
Earth's crust and surface. The rocks are not all the same age-or even nearly
so-but, like the pages in a long and complicated history, they record the Earthshaping events and life of the past. The record, however, is incomplete. Many
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pages, especially in the early parts, are missing and many others are tattered,
torn, and difficult to decipher. But enough of the pages are preserved to reward
the reader with accounts of astounding episodes which certify that the Earth
is billions of years old.
Two scales are used to date these episodes and to measure the age of the
Earth: a relative time scale, based on the sequence of layering of the rocks
and the evolution of life, and the radiometric time scale, based on the natural
radioactivity of chemical elements in some of the rocks. An explanation of
the relative scale highlights events in the growth of geologic science itself;
the radiometric scale is a more recent development borrowed from the
physical sciences and applied to geologic problems.
The Relative Time Scale
At the close of the 18th century, the haze of fantasy and mysticism that tended to obscure the true nature of the Earth was being swept away. Careful
studies by scientists showed that rocks had diverse origins. Some rock layers,
containing clearly identifiable fossil remains of fish and other forms of aquatic
animal and plant life, originally formed in the ocean. Other layers, consisting
of sand grains winnowed clean by the pounding surf, obviously formed as
beach deposits that marked the shorelines of ancient seas. Certain layers are
in the form of sand bars and gravel banks-rock debris spread over the land
by streams. Some rocks were once lava flows or beds of cinders and ash
thrown out of ancient volcanoes; others are portions of large masses of oncemolten rock that cooled very slowly far beneath the Earth's surface. Other
rocks were so transformed by heat and pressure during the heaving and
buckling of the Earth's crust in periods of mountain building that their original
features were obliterated.
Between the years of 1785 and 1800, James Hutton and William Smith advanced the concept of geologic time and strengthened the belief in an ancient
world. Hutton, a Scottish geologist, first proposed formally the fundamental
principle used to classify rocks according to their relative ages. He concluded, after studying rocks at many outcrops, that each layer represented a
specific interval of geologic time. Further, he proposed that wherever uncontorted layers were exposed, the bottom layer was deposited first and was,
therefore, the oldest layer exposed; each succeeding layer, up to the topmost
one, was progressively younger.
Today, such a proposal appears to be quite elementary but, nearly 200 years
ago, it amounted to a major breakthrough in scientific reasoning by
establishing a rational basis for relative time measurements. However, unlike
tree-ring dating-in which each ring is a measure of 1 year's growth-no precise
rate of deposition can be determined for most of the rock layers. Therefore,
the actual length of geologic time represented by any given layer is usually
unknown or, at best, a matter of opinion.
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In places where layers of rocks are contorted, the relative ages of the layers may be difficult to
determine. View near Copiapo, Chile.
William "Strata" Smith, a civil engineer and surveyor, was well acquainted
with areas in southern England where "limestone and shales are layered like
slices of bread and butter." His hobby of collecting and cataloging fossil shells
from these rocks led to the discovery that certain layers contained fossils unlike
those in other layers. Using these key or index fossils as markers, Smith could
identify a particular layer of rock wherever it was exposed. Because fossils
actually record the slow but progressive development of life, scientists use
them to identify rocks of the same age throughout the world.
From the results of studies on the origins of the various kinds of rocks
(petrology), coupled with studies of rock layering (stratigraphy) and the evolution of life (paleontology), geologists reconstruct the sequence of events that
has shaped the Earth's surface. Their studies show, for example, that during
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a particular episode the land surface was raised in one part of the world to
form high plateaus and mountain ranges. After the uplift of the land, the forces
of erosion attacked the highlands and the eroded rock debris was transported
and redeposited in the lowlands. During the same interval of time in another
part of the world, the land surface subsided and was covered by the seas.
With the sinking of the land surface, sediments were deposited on the ocean
floor. The evidence for the pre-existence of ancient mountain ranges lies in
the nature of the eroded rock debris, and the evidence of the seas' former
presence is, in part, the fossil forms of marine life that accumulated with the
bottom sediments.
Such recurring events as mountain building and sea encroachment, of which
the rocks themselves are records, comprise units of geologic time even though
the actual dates of the events are unknown. By comparison, the history of
mankind is similarly organized into relative units of time. We speak of human
events as occurring either B.C. or A.D.-broad divisions of time. Shorter spans
are measured by the dynasties of ancient Egypt or by the reigns of kings and
queens in Europe. Geologists have done the same thing to geologic time by
dividing the Earth's history into Eras -broad spans based on the general
character of life that existed during these times, and Periods -shorter spans
based partly on evidence of major disturbances of the Earth's crust.
The names used to designate the divisions of geologic time are a fascinating
mixture of words that mark highlights in the historical development of geologic
science over the past 200 years. Nearly every name signifies the acceptance
of a new scientific concept-a new rung in the ladder of geologic knowledge.
A paleontologist of the U.S. Geological Survey examining the fossil bones of Paleoparadoxia, an
aquatic mammal that lived about 14 million years ago.
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The major divisions, with brief explanations of each, are shown in the following scale of relative geologic time, which is arranged in chronological order
with the oldest division at the bottom, the youngest at the top.
Major DivisionS-of-Geologic Time
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Quaternary
Period
CENOZOIC ERA
(Age of Recent Life)
The several geologic eras were originally named Primary,
Secondary, Tertiary, and Quaternary. The first two names are
no longer used; Tertiary and Quaternary have been retained
but used as period designations.
Tertiary
Period
MESOZOIC ERA
(Age of Medieval Life)
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PALEOZOIC ERA
(Age of Ancient Life)
Cretaceous
Period
Derived from Latin word for chalk (creta) and first applied
to extensive deposits that form white cliffs along the English
Channel.
Jurassic
Period
Named for the Jura Mountains, located between France and
Switzerland, where rocks of this age were first studied.
Triassic
Period
Taken from word "trias" in recognition of the threefold
character of these rocks in Europe.
Permian
Period
Named after the province of Perm, U.S.S.R., where these
rocks were first studied.
Pennsylvanian
Period
Named for the State of Pennsylvania where these rocks have
produced much coal.
Mississippian
Period
Named for the Mississippi River valley where these rocks are
well exposed.
Devonian
Period
Named after Devonshire, England, where these rocks were
first studied.
Silurian
Period
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Ordovician
Period
Cambrian
Period
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PRECAMBRIAN ERA
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Named after Celtic tribes, the Silures and the Ordovices, that
lived in Wales during the Roman Conquest.
Taken from Roman name for Wales (Cambria) where rocks
containing the earliest evidence of complex forms of life were
first studied.
The time between the birth of the planet and the appearance
of comp lex forms of life. More than 80 percent of the Earth's
estimated 4% billion years falls within this era.
_
Keyed to the relative time scale are examples of index fossils, the forms of
life which existed during limited periods of geologic time and thus are used
as guides to the age of the rocks in which they are preserved.
Index Fossils
PECTEN gibbus
NEPTUNEA tabulata
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VENER ICAR DI A planicosta
SCAPHITES hippocrepis
INOCERAMUS labiatus
PERISPHINCTES tiziani
NERINEA trinodosa
TROPITES subbullatus
MONDTIS subcircularis
PARAFUSULINA bosei
LEPTODUS americanus
DICTYOCLOSTUS americanus
LOPHOPHYLLIDIUM prollferum
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CACTOCRINUS multibrachiatus
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PROLECANITES gurleyi
PALMATOLEPUS unicorniS
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CYSTIPHYLLUM niagarense
BATHYURUS extans
PARADOX IDES pinus
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The following examples show how the rock layers themselves are used as
a relative time scale:
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The Wingate Sandstone is a reddish-brown formation consisting largely of wind-blown sand believed to have accumulated as desert dunes in the Four Corners region of the Southwestern United
States about 200 million years ago. Erosion of this formation commonly produces vertical cliffs.
View near Gateway, Colorado.
The first diagram (page 8) correlates or matches rock units from three
localities within a small area by means of geologic sections compiled from
results of field studies.
These sections are typical of the ones geologists prepare when studying the
relationships of layers of rocks (beds) throughout a region. Each column
represents the sequence of beds at a specific locality. The same beds, wl<lich
in places may thicken or thin (some may pinch out entirely) according to the
local environment of deposition, are bracketed within the lines connecting
the three columns.
For convenience, geologists commonly group adjoining beds that possess
similar or related features (including fossils) into a single, more conspicuous
unit called a formation . The component beds of each formation are described,
the formation is named, and the information is published for the use of all
geologists. Formation names comprise two words, the first one usually taken
from a geographic feature near which the rocks are prominently displayed.
The second word indicates the principal rock type, or if of mixed rock types,
the word formation is used: The Morrison Formation -the Wingate
Sandstone-the Todilto Limestone-the Mancos Shale.
The second diagram (page 10-11) is a composite geologic section, greatly
simplified. This example correlates formations throughout a large region in
the southwestern part of the United States noted for its spectacular scenery.
The composite section can be thought of as representing a large, although
incomplete, segment of relative geologic time. A global time scale has been
derived by extending the correlations from one place to another and reaching
from one continent to another. These correlations are based in large part on
fossil evidence.
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CRETACEOUS PERIOD
JURASSIC PERIOD
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TRIASSIC PERIOD
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Moenkopi Fm
PERMIAN PERIOD
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Coconino Ss
Hermit Shale
PENNSYLVANIAN PERIOD
MISSISSIPPIAN PERIOD
CAMBRIAN PERIOD
PRECAMBRIAN ERA
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Moenkopi Fm
Kaibab ls
Older rocks
not exposed
CANYONLANDS
NATIONAL PARK
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Kaiparowits Fm
Summerville Fm
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Dakota Ss ..
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Older rocks not exposed
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The Radiometric Time Scale
The discovery of the natural radioactive decay of uranium in 1896 by Henry
Becquerel, the French physicist, opened new vistas in science. In 1905, the
British physicist lord Rutherford-after defining the structure of the atommade the first clear suggestion for using radioactivity as a tool for measuring
geologic time directly; shortly thereafter, in 1907, Professor B. B. Boltwood,
radiochemist of Yale University, published a list of geologic ages based on
radioactivity. Although Boltwood's ages have since been revised, they did show
correctly that the duration of geologic time would be measured in terms of
hundreds-to-thousands of millions of years.
The next 40 years was a period of expanding research on the nature and
behavior of atoms, leading to the development of nuclear fission and fusion
as energy sources. A byproduct of this atomic research has been the development and continuing refinement of the various methods and techniques used
to measure the age of Earth materials. Precise dating has been accomplished
since 1950.
A chemical element consists of atoms with a specific number of protons
in their nuclei but different atomic weights owing to variations in the number
of neutrons. Atoms of the same element with differing atomic weights are
A technician of the U.S. Geological Survey uses a mass spectrometer to determine the proportions
of neodymium isotopes contained in a sample of igneous rock .
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called isotopes. Radioactive decay is a sp<;>ntaneous process in which an
isotope (the parent) loses particles from its nucleus to form an isotope of a
new element (the daughter). The rate of decay is conveniently expressed in
terms of an isotope's half-life, or the time it takes for one-half of a particular
radioactive isotope in a sample to decay. Most radioactive isotopes have rapid
rates of decay (that is, short half-lives) and lose their radioactivity within a few
days or years. Some isotopes, however, decay slowly, and several of these
are used as geologic clocks. The parent isotopes and corresponding daughter
products most commonly used to determine the ages of ancient rocks are
listed below:
Parent Isotope
Uranium-238
Uranium-235
Thorium-232
Rubi di um-87
Potass ium-40
Samarium-147
Stable Daughter Product
Lead-206
Lead-207
Lead-208
Strontium-87
Argon-40
Neodymium-143
Currently Accepted
Half-life Values
4.5 billion years
704 million years
14.0 billion years
48.8 billion years
1.25 billion yea rs
106 billion years
The mathematical expression that relates radioactive decay to geologic time
is called the age equation and is: t=
l •tn ~ + ~)
where t is the age of a rock or mineral specimen,
D is the number of atoms of a daughter product today,
P is the number of atoms of the parent isotope today,
ln is the natural logarithm (logarithm to base e), and
A is the appropriate decay constant.
(The decay constant for each parent isotope is related to its
half-life, t%, by the following expression: t1/2 =
1 2
~ )
Dating rocks by these radioactive timekeepers is simple in theory, but the
laboratory procedures are complex. The numbers of parent and daughter
isotopes in each specimen are determined by various kinds of analytical
methods. The principal difficulty lies in measuring precisely very small amounts
of isotopes.
The potassium-argon and argon-argon methods can be used on rocks as
young as a few thousand years as well as on the oldest rocks known. Potassium
is found in most rock-forming minerals, the half-life of its radioactive·isotope
potassium-40 is such that measurable quantities of argon (daughter) have accumulated in potassium-bearing minerals of nearly all ages, and the amounts
of potassium and argon isotopes can be measured accurately, even in very
small quantities. Where feasible, two or more methods of analysis are used
on the same specimen of rock to confirm the results.
Another important atomic clock used for dating purposes is based on the
radioactive decay of the isotope carbon-14, which has a half-life of 5,730 years.
Carbon-14 is produced continuously in the Earth's upper atmosphere as a
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Carbon samples are converted to acetylene gas by combustion in a vacuum line. The acetylene
gas is then analyzed in a mass spectrometer to determine its carbon isotopic composition.
result of the bombardment of nitrogen by neutrons from cosmic rays. This
newly formed radiocarbon becomes uniformly mixed with the nonradioactive carbon in the carbon dioxide of the air, and it eventually finds its way
into all living plants and animals. In effect, all carbon in living organisms contains a constant proportion of radiocarbon to non-radioactive carbon. After
the death of the organism, the amount of radiocarbon gradually decreases
as it reverts to nitrogen-14 by radioactive decay. By measuring the amount
of radioactivity remaining in organic materials, the amount of carbon-14 in
the materials can be calculated and the time of death can be determined. For
example, if carbon from a sample of wood is found to contain only half as
much carbon-14 as that from a living plant, the estimated age of the old wood
would be 5,730 years.
The radiocarbon clock has become an extremely useful and efficient tool
in dating the important episodes in the recent prehistory and history of man,
but because of the relatively short half-life of carbon-14, the clock can be used
for dating events that have taken place only within the past 50,000 years.
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The following is a .group of rocks and materials that have been dated
by various atomic clock methods:
Approximate Age
Sample
in Years
Cloth wrappings from a mummified bul l .. ..................... .. .... ... .. .. ....................... .. .............. .. ... 2,050
Samples taken from a pyramid in Dashur, Egypt. This date agrees with the age
of the pyramid as estimated from historical records.
Charcoal ........ ........ ....... ..................... .. .. ....... ............................... ........ ......................... ...... .. .. .... 6,640
Sample, recovered from bed of ash near Crater Lake, Oregon, is from a tree
burned in the vio lent eruption of Mount Mazama which created Crater Lake.
This eruption blanketed several States with ash, providing geologists with an
excellent time zone.
Charcoal . .. ................................................. .. .. .. ............................................... .......................... 10,130
Sample collected from the " Marmes Man " site in southeastern Washington.
This rock shelter is believed to be among the old est known inhabited sites in
North Ameri ca.
Spruce wood .............. ..... .... ......... .. .. ............................. ........... .. .... ........... .. .............................11,640
Sample from the Two Creeks forest bed near Milwaukee, Wi scon sin, dates
on e of th e last advances of th e continental ice sheet into the United States.
Bishop Tu ff ........................................................................ ... ................................. .... ... ....... ...700,000
Samples collected from vol canic ash and pumice that overlie glacial debris in
Owen s Valley, California, Thi s vol canic episode provid es an important
reference datum in th e glacial hi story of North America.
Vol canic ash ................................. ... ..................... ....... ... .... ...... ... .. .. ............ ................ ........1,750,000
Samples collected from strata in Olduvai Gorge, East Africa, which sandwich
the fossil remains of Zinjanthropu s and Homo Habilis-possible precursors of
modern man .
Monzonite............................................................................................................... .. .. .......37,500,000
Samples of copper-bearing rock from vast open-pit mine at Bingham Canyon,
Utah.
Quartz monzonite............................................................................................................. ao,ooo,ooo
Samples collected from Half Dom e, Yosemite National Park, California.
Conway Granite ............................................................... .................... ............ .. .. .... ........ 180,000,000
Samples collected from Red ston e Quarry in the White Mountains of New
Hampshire.
Rhyolite . ............. ........... ....... .. ........ ... .............. ... ............................... ............................... 820,000,000
Samples collected from Mount Rogers, the highest point in Virginia.
Pikes Peak Granite........ ....... ... .. ... ... .. .. ....... ............ .. ...... .. ........................................... .. 1,030,000,000
Samples collected on top of Pikes Peak, Colorado.
Gnei ss .... ... .. ................. ...... ... ... .... .. ... .............. .. ............................................................. 2,700,000,000
Samples from outcrops in the Karelian area o f eastern Finland are
believed to repres ent th e oldest rocks in the Baltic region .
The Old Granite ... .... .. ... ........................................... ............ ... ... ....... .. ........................ .. 3,200,000,000
Samples from outcrops in the Tran svaal, South Africa. These rocks
intrude even older rocks that have not been dated .
Morton Gneiss ................... .................... .... ... .... .. .. ................ .. ........................... .... .. .....3,600,000,000
Samples from outcrops in southwestern Minnesota are believed to
represent the oldest rocks in North America.
15
Interweaving the relative time scale with the atomic time scale poses certain probtems because only certain types of rocks, chiefly the igneous variety, can be dated directly by radiometric methods; but these rocks do not
ordinarily contain fossils. Igneous rocks are those such as granite and basalt
which crystallize from molten material called "magma".
When igneous rocks crystallize, the newly formed minerals contain various
amounts of chemical elements, some of which have radioactive isotopes. These
isotopes decay within the rocks according to their half-life rates, and by selecting the appropriate minerals (those that contain potassium, for instance) and
Rocks of
Rocks of
Jurassic Age
16
measuring the relative amounts of parent and daughter isotopes in them, the
date at which the rock crystallized can be determined. Most of the large igneous rock masses of the world have been dated in this manner.
Most sedimentary rocks such as sandstone, limestone, and shale are related
to the radiometric time scale by bracketing them within time zones that are
determined by dating appropriately selected igneous rocks, as shown in the
hypothetical example on page 16.
The age of the volcanic ash bed and the igneous dike are determined directly
by radiometric methods. The layers of sedimentary rocks below the ash bed
are obviously older than the ash, and all the layers above the ash are younger.
The igneous dike is younger than the Mancos Shale and Mesaverde Formation but older than the Wasatch Formation because the dike does not intrude
the Tertiary rocks.
From this kind of evidence, geologists estimate that the end of the
Cretaceous Period and the beginning of the Tertiary Period took place between 60 and 70 million years ago. Similarly, a part of the Morrison Formation of jurassic age was deposited about 160 million years ago as indicated
by the age of the ash bed.
Literally thousands of dated materials are now available for use to bracket
the various episodes in the history of the Earth within specific time zones.
Many points on the time scale are being revised, however, as the behavior
of isotopes in the Earth's crust is more clearly understood. Thus the graphic
illustration of the geologic time scale, showing both relative time and
radiometric time, represents only the present state of knowledge. Certainly,
revisions and modifications will be forthcoming as research continues to improve our knowledge of Earth history.
The Age of the Earth
The oldest known formation in North America, the Morton Gneiss of Minnesota, appears to have formed about 3112 billion years ago. Other rocks of
similar or somewhat younger age occur in South Africa, Finland, Australia,
and elsewhere. Upon what evidence is a minimum age for the Earth of 4Yz
billion years based?
Some estimates of the age of the Earth are based on the decay rates of
radioactive isotopes. One such estimate is based on two long-lived isotopes,
uranium-238 and uranium-235, and places the maximum age of the chemical
elements that make up the Earth at about 6 billion years. The reasoning behind
this estimate involves theoretical concepts about the origin of uranium which
stipulate that not more than half the first-formed uranium consisted of the
uranium-235 isotope. But, because of the changes in isotopic composition that
have taken place throughout geologic time as a result of radioactive decay,
uranium today contains only about 0.7 percent of the uranium-235 isotope.
17
Calculations based on the known decay rates for these two isotopes (table,
page 13) indicate that a span of about 6 billion years is required for the isotopic
composition of uranium to have changed from 50 percent to 0.7 percent
uranium-235. Had uranium originally consisted of more than half of the
uranium-235 isotope, fhe calculated age of uranium would exceed 6 billion
years.
Another estimate of the Earth's age is based on the progressive buildup in
the Earth's crust of lead isotopes that have been derived from radioactive decay
of uranium-238, uranium-235, and thorium-232. Studies of the relative abundance of these "radiogenic" leads suggest that the Earth is not older than 5.5.
billion years.
An approximate age for the Earth has been determined from studies of
meteorites. Meteorites are specimens of matter from space; they are classified
according to their chemical composition into three main groups: the irons,
the stony irons, and the stones. Because of chemical similarities between
meteorites and the rocks of the Earth (these three groups may correspond
to the three major parts of the Earth-the core, the mantle, and the crust),
it is generally assumed that meteorites and the Earth have a common origin
and were formed as solid objects at about the same time during the evolution of the Solar System. Thus, what can be learned from meteorites can be
used to interpret the Earth. Chemical analyses of iron meteorites show that
the uranium content is so low that the isotopic composition of lead has not
been changed significantly by the addition of radiogenic lead since the formation of meteorites. Thus,the lead in iron meteorites is considered to be
primordial lead. Stony meteorites, however, contain sufficient uranium to have
produced appreciable quantities of radiogenic lead since the meteorites were
formed. When the measured amounts of radiogenic lead in stony meteorites
are corrected by subtracting the amount of primordial lead in iron meteorites,
calculations yield an age of about 41h billion years for stony meteorites. If the
Earth and meteorites have a common or a similar origin, then it seems
reasonable to assume that the age of the Earth is about the same as the age
of the meteorites.
Thus, as a current working hypothesis based on several kinds of evidence,
geologists consider the Earth to be at least 41h billion years old.
This publication is one of a series of general interest publications prepared by the U.S. Geological
Survey to provide information about the earth sciences, natural resources, and the environment. To
obtain a list of additional titles in the series "General Interest Publications of the U.S. Geological Survey,"
write:
Book and Open-File Reports Section
U.S. Geological Survey
Federal Center, Box 25425
Denver, CO 80225
Printed 1988
20
As the Nat io n's princi pal con servatio n agency, th e Department of the Interior has th e respo nsibility fo r most of o ur natio nally owned publi c land s and natural resources. Thi s incl ud es foste ring the w isest use of o ur land and wate r resources, protec tin g our fish and w ildli fe, prese rving
the environm ental and cultural valu es of o ur natio nal park s and histori cal places, and providin g
for the enjoyment of life through outdoor recreation. The Deparment assesses our energy and min eral resources
and wo rks to ass ure that th eir develo pm ent is in th e best interes ts of all o ur peopl e. The Department also
has a majo r respo nsibili ty fo r American Indi an reservatio n co mmun ities and fo r peo pl e w ho live in Island Terri tor ies und er U.S. administratio n.